[defcon65]

I would guess these would be ordinary split-ring lockwashers. You might try the stainless steel versions as seat bolts have a tendency to corrode from moisture coming through the bottom of the pan and from carpet cleaners, spilled beverages, snowy, wet feet, etc. I have applied a little grease to the threads when reinstalling bolts in this location to keep them from seizing up.

Hope that helps!